A loudspeaker is one of the most basic and key output devices in any interactive system. It is a transducer that converts an input electrical signal into an audible acoustic signal. The most common approaches to designing speakers are electromagnetic and piezoelectric speakers, and both approaches have a number of important limitations.
Electromagnetic speakers include a voice coil and a magnet, and the sound is generated by the vibrations of the paper cone induced by moving the magnet. Electromagnetic speakers are relatively large and consist of multiple materials and moving parts. The shape of the electromagnetic speaker is usually limited to a classic cone or its variations. Although mass-produced speakers are relatively cheap, designing and producing custom speakers is an order of magnitude more expensive and requires significant engineering efforts.
Piezoelectric speakers usually consist of two electrodes with a thin piezoelectric element (PZT), such as lead zirconate titanate, sandwiched in between. As a signal is applied to the electrodes the PZT element bends, creating audible vibration. Although piezoelectric speakers are simple and inexpensive, they are produced by baking piezoelectric paste at very high temperatures, and therefore it is difficult and expensive to produce them in anything other than a flat shape, particularly in small quantities. Increasing the size of the PZT elements is particularly challenging because their response rapidly decreases with increased size and thickness. Another important property of PZT speakers is that they are capable of creating ultrasonic sound sources and they are commonly used in sensor design.
A less commonly used technology for sound production is electrostatic loudspeaker technology (ESL), which had been intensively investigated in the early 1930s through the 1950s.